1. Field of the Invention
The invention relates to the fabrication of ballistic resistant fibrous composites having improved ballistic resistance properties. More particularly, the invention pertains to ballistic resistant fibrous composites having high lap shear strength between component fiber layers, which correlates to low composite backface signature. The high lap shear strength composites are particularly useful for the production of hard armor articles, including helmets meeting current National Institute of Justice (NIJ) backface signature requirements.
2. Description of the Related Art
Ballistic resistant articles fabricated from composites comprising high strength synthetic fibers are well known. Articles such as bullet resistant vests, helmets, vehicle panels and structural members of military equipment are typically made from fabrics comprising high strength fibers such as SPECTRA® polyethylene fibers or Kevlar® aramid fibers. For many applications, such as vests or parts of vests, the fibers may be used in a woven or knitted fabric. For other applications, the fibers may be encapsulated or embedded in a polymeric matrix material and formed into non-woven fabrics. For example, U.S. Pat. Nos. 4,403,012, 4,457,985, 4,613,535, 4,623,574, 4,650,710, 4,737,402, 4,748,064, 5,552,208, 5,587,230, 6,642,159, 6,841,492, 6,846,758, all of which are incorporated herein by reference, describe ballistic resistant composites which include high strength fibers made from materials such as extended chain ultra-high molecular weight polyethylene (“UHMW PE”). Ballistic resistant composites fabricated from such high strength synthetic fibers exhibit varying degrees of resistance to penetration by high speed impact from projectiles such as bullets, shells, shrapnel and the like, as well as varying degrees of backface signature resulting from the same projectile impact.
It is known that each type of high strength fiber has its own unique characteristics and properties. In this regard, one defining characteristic of a fiber is the ability of the fiber to bond with or adhere with surface coatings, such as resin coatings. For example, ultra-high molecular weight polyethylene fibers are relatively inert, while aramid fibers have a high-energy surface containing polar functional groups. Accordingly, resins generally exhibit a stronger affinity aramid fibers compared to the inert UHMW PE fibers. Nevertheless, it is also generally known that synthetic fibers are naturally prone to static build-up and thus typically require the application of a fiber surface finish in order to facilitate further processing into useful composites. Fiber finishes are employed to reduce static build-up, and in the case of untwisted and unentangled fibers, to aid in maintaining fiber cohesiveness. Finishes also lubricate the surface of the fiber, protecting the fiber from the equipment and protecting the equipment from the fiber. The art teaches many types of fiber surface finishes for use in various industries. See, for example, U.S. Pat. Nos. 5,275,625, 5,443,896, 5,478,648, 5,520,705, 5,674,615, 6,365,065, 6,426,142, 6,712,988, 6,770,231, 6,908,579 and 7,021,349, which teach spin finish compositions for spun fibers.
However, typical fiber surface finishes are not universally desirable. One notable reason is because a fiber surface finish can interfere with the interfacial adhesion or bonding of polymeric binder materials on fiber surfaces, including aramid fiber surfaces. Strong adhesion of polymeric binder materials is important in the manufacture of ballistic resistant fabrics, especially non-woven composites such as non-woven SPECTRA SHIELD® composites produced by Honeywell International Inc. of Morristown, N.J. Insufficient adhesion of polymeric binder materials on the fiber surfaces may reduce fiber-fiber bond strength and fiber-binder bond strength and thereby cause united fibers to disengage from each other and/or cause the binder to delaminate from the fiber surfaces. A similar adherence problem is also recognized when attempting to apply protective polymeric compositions onto woven fabrics. This detrimentally affects the ballistic resistance properties (anti-ballistic performance) of such composites and can result in catastrophic product failure.
The anti-ballistic performance of composite armor can be characterized in different ways. One common characterization is the V50 velocity, which is the experimentally derived, statistically calculated impact velocity at which a projectile is expected to completely penetrate armor 50% of the time and be completely stopped by the armor 50% of the time. For composites of equal areal density (i.e. the weight of the composite panel divided by the surface area) the higher the V50 the better the penetration resistance of the composite. However, even when anti-ballistic armor is sufficient to prevent the penetration of a projectile, the impact of the projectile on the armor may also cause significant non-penetrating, blunt trauma (“trauma”) injuries. Accordingly, another important measure of anti-ballistic performance is armor backface signature. Backface signature (“BFS”), also known in the art as backface deformation or trauma signature, is the measure of the depth of deflection of body armor due to a bullet impact. When a bullet is stopped by composite armor, potentially resulting blunt trauma injuries may be as deadly to an individual as if the bullet had penetrated the armor and entered the body. This is especially consequential in the context of helmet armor, where the transient protrusion caused by a stopped bullet can still cross the plane of the wearer's skull and cause debilitating or fatal brain damage.
It is known that the V50 ballistic performance of a composite is directly related to the strength of the constituent fibers of the composite. Increases in fiber strength properties such as tenacity and/or tensile modulus are known to correlate with an increase in V50 velocity. However, a corresponding improvement of backface signature reduction with increased fiber strength properties has not been similarly recognized. Accordingly, there is a need in the art for a method to produce ballistic resistant composites having both superior V50 ballistic performance as well as low backface signature. The invention provides a solution to this need.
It has been unexpectedly found that there is a direct correlation between backface signature and the tendency of the component fibers of a ballistic resistant composite to delaminate from each other and/or delaminate from fiber surface coatings as a result of a projectile impact. By improving the bond between a fiber surface and a fiber surface coating, the fiber-fiber disengagement and/or fiber-coating delamination effect are reduced, thereby increasing friction on the fibers and increasing projectile engagement with the fibers. Accordingly, the composite structural properties are improved and the energy of a projectile impact is dissipated in a manner that reduces the composite backface deformation.
The invention addresses this need in the art by processing the fibers to improve the bond between a fiber surface and a fiber surface coating prior to uniting the fibers as non-woven fiber layers or fabrics, or prior to weaving fibers into woven fabrics, and prior to coating the fibers with select polymers, as well as prior to merging multiple fiber layers into a multi-ply or multi-layer composite. It has been found that fibrous composites formed from such treated fibers have improved interlaminar lap shear strength between adjoined fiber plies/layers of a multi-ply/multi-layer fibrous composite. Particularly, the fibers are processed to remove at least a portion of the fiber surface finish to expose at least a portion of the fiber surface, thereby allowing a subsequently applied polymer to bond directly with the fiber surface such that the polymer is predominantly in direct contact with the fiber surface rather than predominantly atop the finish. A variety of other fiber treatments may also be conducted to further enhance the ability of a subsequently applied material to adsorb to, adhere to or bond to the fiber surface. The higher lap shear strength reflects increased fiber-fiber bonding within a single fiber ply, increased ply-ply bonding within a single multi-ply fabric or multi-ply fiber layer, and correlates to improved composite structural properties as well as improved composite backface signature.